The present invention relates to a radiological measurement system and a radiological imaging system.
As a detector in the radiological measurement system, there are an ionization chamber, a scintillation detector, a semiconductor detector, and a cumulative radiation monitor. They are used properly according to the use. If it is desired to obtain not only a dose of gamma rays or X-rays but also its energy information, then the semiconductor detector is typically used. If gamma rays or X-rays are incident on a semiconductor, then the gamma rays or X-rays undergo energy conversion and a large amount of carrier charges such as electrons or holes are generated and the charge quantity is proportional to energy of the gamma rays or X-rays. In other words, since the energy can be known by measuring the charge quantity, high precision measurement results are obtained.
In the scintillation detector as well, energy can be found by converting gamma rays or X-rays to light and then measuring the quantity of light with a photo-multiplier tube or a photodiode in the same way. Since gamma rays or X-rays are converted to light once, however, it is difficult to obtain high precision energy information. In many cases, therefore, the above-described semiconductor detector is used exclusively.
As a material used as the semiconductor detector, silicon, germanium, cadmium telluride and cadmium zinc telluride are known. Silicon is a material which is used in ICs as well and which is best known. Since silicon is as small as 14 in atomic number and low in density, however, the probability that gamma rays or X-rays react in crystal is low and silicon is limited to low energy use such as elemental composition analyzers.
Germanium has a little large atomic number 32, large carrier mobility, and extremely fine energy discrimination ability (energy resolution), and germanium is used as detectors as well. Since germanium is as small as approximately 0.7 eV in band gap, however, its leakage current is large at the room temperature and germanium needs to be cooled to low temperatures in use.
Cadmium telluride and cadmium zinc telluride are approximately 1.4 eV in band gap and small in leak current even at the room temperature. Therefore, cadmium telluride and cadmium zinc telluride can be used suitably as a radiation detector. Furthermore, cadmium telluride and cadmium zinc telluride are as large as 50 in average atomic number and high in sensitivity to gamma rays or X-rays as well. Cadmium zinc telluride uses elements of three kinds, and it is difficult to obtain crystal with a high yield. However, cadmium telluride formed of elements of two kinds is a material which is comparatively good in yield and which has become possible to mass-produce in recent years.
Cadmium telluride is a material which is excellent as the radiation detector as described above. In general, a diode is formed of cadmium telluride and supplied with a reverse bias voltage in use.
A problem called polarization effect occurs when cadmium telluride is used. This is a phenomenon caused by continuing to apply a bias voltage continuously in order to conduct radiological measurement especially in a radiation detector using cadmium telluride for a diode. This is a phenomenon that the energy resolution is aggravated and the sensitivity is also lowered remarkably. Its cause is interpreted to be that carriers are trapped by crystalline defects and stored as space charges.
This is a phenomenon well known in handling the radiation detector which uses cadmium telluride as described in, for example, Non-Patent Document 1. This phenomenon is not so remarkable at the room temperature or below. If the room temperature is exceeded, this phenomenon advances in the order of several minutes to several tens minutes. The radiation detector is typically used in a place located near an electronic circuit such as a detected signal amplifier. Unless cooled, therefore, the temperature becomes a little higher than the room temperature. Conversely, unless the radiation detector is stable at a little higher temperature than the room temperature, it is difficult to use the radiation detector as a detector. If cooling is conducted, then the problem of the polarization effect can be avoided, but a different problem such as complication caused by a countermeasure against dewfall or a cooling mechanism occurs. Unless the radiation detector using cadmium telluride can be used without cooling, therefore, advantages also go down as compared with the case of germanium described earlier.
As a measure for avoiding the polarization effect besides cooling, a method of making the bias voltage zero temporarily is known. This can be implemented by using, for example, a method described in Patent Document 1. In Patent Document 1, however, a circuit is shown supposing that the bias voltage is changed in approximately several seconds. While the bias voltage is zero or changing, radiological measurement is not conducted (this time period is called dead time). As a result, a problem that continuity of the measurement is not kept is posed. If the dead time is at least several seconds, the continuity of the measurement is not kept. Therefore, a radiological measurement situation in which the discontinuity of the measurement is allowed becomes comparatively limitative. In other words, use for which the radiation detector can be used is limited unadvantageously.
As measurement conditions and a measure for shortening a time period required to make the bias voltage equal to zero, there are those disclosed in Patent Document 2. According to Patent Document 2, a sufficient effect is obtained for suppression of the polarization effect by setting the time period required to make the bias voltage zero equal to approximately 0.5 to 1.0 second and setting its space to approximately 5 minutes. Nevertheless, the time period over which the bias voltage is zero or changing occurs for approximately one second, and it can be said by no means that the time period is short.
Therefore, the present inventors studied how short the time period required to make the bias voltage zero can be made without exerting an influence upon the avoidance of the polarization effect. As a result, the present inventors have found that the time period over which the bias voltage is made zero can be made equal to approximately several milliseconds provided that the space between the time periods over each of which the bias voltage is made zero is set equal to approximately several tens seconds. If the dead time is approximately several milliseconds, then the time is sufficiently short and consequently the continuity of the measurement is not hampered in many apparatuses. For apparatuses using a radiation detector, such as the gamma camera, single photon emission CT apparatus (SPECT), positron emission CT apparatus (PET) and environment radiation monitoring system, the dead time of approximately several milliseconds is a sufficiently short time.
If the time period over which the bias voltage is made zero is the order of milliseconds, i.e., the bias voltage is turned on and off in the order of milliseconds, however, then a surge current flows and there is a fear that the surge current will enter an amplifier and damage the amplifier. However, this can be avoided by providing the well-known protection circuit using diodes or the like on an input side of the amplifier. If circuit elements are selected carefully while paying attention to capacitance of the diodes used in the protection circuit, then it is possible to protect the amplifier almost without causing degradation in performance such as energy resolution.
Patent Document 1: JP-A-2004-138472
Patent Document 2: JP-B-3938189 (corresponds to US2006/0138336)
Non-Patent Document 1: IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 45, ISSUE 3, PART 1, PP. 428-432 (1998)